Air guns use compressed air to discharge a pellet. Some air guns use a piston in a compression chamber. A spring is mounted between the piston and a trigger mechanism. The piston is retracted to cock the trigger and to compress the spring. When the trigger is released, the spring biases the piston toward the barrel to compress the air in the compression chamber. The compressed air then propels the pellet through the barrel. One problem with air guns using a mechanical spring is that the spring tends to fatigue or break, and also to produce an excessive amount of spring vibration upon firing.
Some air guns use a gas spring instead of a mechanical spring. The piston is moved toward the trigger to compress a gas or air behind the piston. When the trigger is released, the piston is driven forward by the compressed gas thereby compressing the air in front of the piston to discharge the pellet out of the barrel.
Such guns are illustrated in U.S. Pat. No. 4,709,686 which was issued Dec. 1, 1987 to Hugh F. Taylor and David R. Theobald for "Air Weapon With Gas-Tight Expansion Chamber"; U.S. Pat. No. 4,771,758 which was issued Sep. 20, 1988 to the same inventors for "Air Weapon With Air Compression System Having Grooves For Air Transfer"; and U.S. Pat. No. 4,850,329 which was issued to Jul. 25, 1989 to Hugh F. Taylor, David R. Theobald and Derek J. C. Bernard for "Firing Mechanisms For Air Weapons".